In the fabrication of integrated circuits the silicon wafers on which circuits are formed must under go a large number of processing steps. For example, each wafer must be coated, exposed, developed, etched and recoated many times as the integrated circuits on the wafers are built up layer by layer. For facilitating alignment and movement of the wafer through the processing steps, the wafer is mounted on a wafer chuck assembly and brought into contact with an X, Y, .theta. alignment plate. The X, Y, .theta. plate is continuously supported by precision air bearings to provide substantially zero-friction operation necessary for sub-micron adjustments. Typically the plate is driven in the X, Y, .theta. coordinates by 3 stepping motors linked to the plate by appropriate gear packages and linkage assemblies. When correct alignment has been established, brakes are applied to lock the plate in the desired position.
An object of the present invention is to apply a small known, repeatable, force to the wafer chuck assembly during the establishment of a vacuum between the X, Y, .theta. plate and the wafer chuck assembly. The device and method of the present invention eliminates the many difficulties previously encountered in controlling the magnitude and adjustment of the forces necessary to obtain a proper wafer chuck hold-down operation.
Another object of the invention is to reduce the hold-down time, thereby increasing the wafer throughput.